Saturday, January 23, 2010

Extreme Myths About Microprocessor.

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In every single day we come across many intelligent devices that has a microprocessor embedded in it. People get passionate over processors more than RAM or Decoders. Every one has their own favorite processor which they like to recommend others and also to use over and over again. Legend and lore surrounds Microprocessor. Some are true but many are just myths which is believed to be true.

  • Few Processor Choices.
This is the most subtle misconception while designing an embedded system that, we are having a choice of only 10-20 32-bit processors. But the fact is there are 100 different 32bit processors available for sale. This counting is not irrespective of different packages and speed; but dozen of companies make different CPU architecture and instruction sets.

  • Intel Rules The World Of Microprocessors.
For lot of people microprocessor means Intel Pentium. The newspaper proclaims that Intel has a 95% share of microprocessor market. This are all media hypes. Intel Pentium has a dominant share of the PC business. But PC contributes to a very small slice of the Microprocessor pie. 
In 98% of the embedded CPU's Intel is not even in the top 5. ARM vendors alone sell about three times of the Intel sells Pentium.
  • Instruction Sets Don't Matter.
Instruction set affect your assembly codes. We go on using instructions as it is meant. We have never thought of Instruction Sets in this context. Whether you use Java, C/C++ or Ada it is ultimately dumped into the hardware. Performance, predictability and power consumption are heavily dependent on the underlying instruction set.
Let us take an example of multiplying two numbers. This is a very easy job and hardly any programmer will worry. But different chips handle multiplication in different ways. Many RISC chip couldn't even do multiplication. So the early RISC compilers had to synthesize their own integer multiplication function. But  it wasn't fast operation. Now the processors have a built in multiply instruction, but not all multipliers are the same. Some chips can multiply two numbers much faster, than the others and it has nothing to do with clock frequency. 
Figure illustrates this phenomenon:-

Hitachi's SH7604 and SH7708 can multiply 32 numbers in four cycles whereas Motorola's 68020 and 68030 take more than forty cycles to do the same.
  • RISC Is Better Than CISC
Both RISC and CISC have their strength and neither is better all the time. RISC chip provides more code density, smaller memory footprints and more mature software tools. But RISC chips have higher clock rates. So the selection clearly depends on your application.

  • JAVA Chips Are Coming.
Java language was never meant to be handled in hardware. Garbage collection, threads, stack orientation, and object management take a megabyte worth of Java virtual machine to translate into something that even the fastest computer of today struggles to execute. Decades of computer evolution and research in companies and universities around the world has failed to produce any thing like Java machine. But java accelerator chips are available. It executes 30% to 60% of Java Bytes into hardware.
  • Dhrystone-MIPS Are A Useful Benchmark.
To understand what DMIPS are; you will first have to understand what exactly a Dhrystone is.  Dhrystone is a synthetic computing benchmark program which represents the general processor performance. The Dhrystone counts only the program iteration completion per second. When Dhrystone is divided by 1,757 then it is represented as DMIPS.(Dhrystone million instruction per second.)
As the instructions are not same from processor to processor so counting and comparing them is meaningless. For Example a same high level task may require more instructions on a RISC machine but may get execute as a single CISC instruction.  
  • Arm Consumes Lowest Power.
In the early ninety's the arm was one of the first 32 bit processor to be embedded into the ASIC's rather than soldered alongside as a separate chip. It was possible because ARM had no cache, no drivers, no outside bus and not much of an instruction set.
But today, there are plenty of 32 bit processors available as ASIC's core. They use even less power both in standby or when they are active.  
  • Higher The Performance Higher The Price.
Cost it what  it is required to built a chip but price is what marketing department wants it to be. Price is determined by market forces and there is no such relation between price and Performance.

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